1. Field of the Invention
The present invention relates to a two degree of freedom position control method, two degree of freedom position control device and medium storage device using both feedback and feed forward, and more particularly to a two degree of freedom position control method, two degree of freedom position control device and medium storage device for performing two degree of freedom control using observer control.
2. Description of the Related Art
A device for controlling the position of an object to a target position is widely used. For example, this device is used for seek control for moving a head to a target track, which is one of the position controls of a disk device, such as a magnetic disk device and an optical disk device.
A method for this seek control is using two degree of freedom control. In the two degree of freedom control, a target position is provided via a filter while forming a feedback loop. In other words, the transfer function from a target position to an observation position is set to be a form of a low pass filter. By this, overshoot (overrun) can be suppressed effectively.
In the observer control system which is generally used for disk devices as well, a two degree of freedom control system can be constructed. In this case, the control system has a form of a second degree low pass filter (hereafter called LPF), which has a same pole as the feedback pole. FIG. 15 is a block diagram of a two degree of freedom control system of a prior art, and FIG. 16 is a diagram depicting a target trajectory.
As FIG. 15 shows, a target trajectory generation section 100 calculates a seek distance from a target position ‘r’, and generates a target trajectory r (n) according to a seek distance Lseek. As FIG. 16 shows, the target trajectory r (n) indicates a target position to which an object is moved for each sample. A position error computing section 102, on the other hand, computes an error y (n) between the target position ‘r’ and a current position ‘y’ observed from a plant 106. A controller 104 receives the target trajectory r (n) and the position error y (n), performs the computation of the two degree of freedom observer, calculates a drive command value of the plant 106, and drives the plant 106 (e.g. “Digital Control of Dynamic Systems”, (by Gene F. Franklin and two others, published by Addison-Wesley, 1998)).
In this seek control using two degree of freedom control, it is necessary to maintain a position error for a state variable of an observer. On the other hand, the recording density of a medium storage device, represented by a magnetic disk device, is radically increasing recently. For example, in a 2.5 inch HDD (Hard Disk Drive), the capacity per disk has reached 60 GB. The number of tracks is also rapidly increasing accordingly, and a 2.5 inch HDD, for example, has 40 to 50 thousand tracks.
In such a device, a decrease in cost and the downsizing of the device are demanded. Therefore, not a floating point but a fixed point MCU (Microcontroller Unit) is used. In particular, a 16-bit computing unit is widely used.
As FIG. 17 shows, in the 16-bit fixed point method, sign bit ‘S’ is set in a most significant bit, and a number is represented by the remaining 15 bits. Since 16 bits represent only ‘−32768’ to ‘+32678’, it is impossible to represent a number such as the above mentioned 50,000 with a sign.
As a method to extend the number of digits in this fixed point method, a Q-format method is known. As FIG. 18 shows, in the case of the Q2-format, for example, the significant 2 bits are used for an integer, and the insignificant 14 bits are used for decimal. In the case of the Q-1-format, the position of a decimal point is set to minus 1 LSB (least Significant Bit), so as to extend the number of bits to represent a number to 16 bits. In this case, one bit above the decimal point cannot be represented.
If this conventional method of extending the number of digits is applied to the above mentioned two degree of freedom observer, the accuracy of an observed position and estimated position drops because the position of the decimal point is set to minus. For example, if the position of the decimal point is minus 1 LSB, then the insignificant 1 bit above the decimal point is rounded.
This deteriorates calculation accuracy, and the fluctuation of seek current in the position control increases, particularly in the acceleration period, and seek current does not flow smoothly, which causes resonance and noise. As the recording density further increases and the number of tracks increases in the future, this influence becomes progressively more serious.